Metallic nanoparticles, having a diameter of about 1-100 nanometers (nm), are important materials for applications that include semiconductor technology, magnetic storage, electronics fabrication, and catalysis. Metallic nanoparticles have been produced by gas evaporation; by evaporation in a flowing gas stream; by mechanical attrition; by sputtering; by electron beam evaporation; by thermal evaporation; by electron beam induced atomization of binary metal azides; by expansion of metal vapor in a supersonic free jet; by inverse micelle techniques; by laser ablation; by laser-induced breakdown of organometallic compounds; by pyrolysis of organometallic compounds; by microwave plasma decomposition of organometallic compounds, and by other methods.
It is known that metallic nanoparticles possess unique optical properties. In particular, metallic nanoparticles display a pronounced optical resonance. This so-called plasmon resonance is due to the collective coupling of the conduction electrons in the metal sphere to the incident electromagnetic field. This resonance can be dominated by absorption or scattering depending on the radius of the nanoparticle with respect to the wavelength of the incident electromagnetic radiation. Associated with this plasmon resonance is a strong local field enhancement in the interior of the metal nanoparticle. A variety of potentially useful devices can be fabricated to take advantage of these specific optical properties. For example, optical filters or chemical sensors based on surface enhanced Raman scattering (SERS) have been fabricated.
U.S. Pat. No. 6,344,272 (Oldenburg et al.) describes nanoparticles comprised of a nonconducting inner layer that is surrounded by an electrically conducting material. The ratio of the thickness of the nonconducting layer to the thickness of the outer conducting shell is determinative of the wavelength of maximum absorbance or scattering of the particle. The reference notes that a serious practical limitation to realizing many applications of solid metal nanoparticles is the inability to position the plasmon resonance at desired wavelengths. For example, solid gold nanoparticles of 10 nm in diameter have a plasmon resonance centered at 520 nm. This plasmon resonance cannot be controllably shifted by more than approximately 30 nanometers by varying the particle diameter or the specific embedding medium.
Surface Plasmon Resonance (SPR) is the resonant excitation of oscillating free charges at the interface of a metal and a dielectric. When SPR spectra are generated and collected, they can be used to determine specificity, kinetics, affinity, and concentration with respect to the interactions between two or more molecules, where one of the molecules is attached to a solid sensing surface. Reaction kinetics corresponds to both an association and a dissociation rate at which an analyte interacts with the bound detection molecule. Affinity refers to the strength with which an analyte binds to the detecting molecule. Specificity refers to the propensity of a molecule to bind to the detecting molecule to the exclusion of other molecules. SPR spectra have been used in studies involving many types of molecules including proteins, peptides, nucleic acids, carbohydrates, lipids, and low molecular weight substances (e.g., hormones and pharmaceuticals).
Recently, sensor devices have been developed in the known art to exploit the unique optical properties of these nanoparticles. SPR measurements have been made using gold nanoparticle suspensions to detect biomolecular interactions in real time by monitoring the absorbance of colloidal suspensions.
One analytical technique, known as SPR based bio-sensing, has been developed to enable direct measurements of the association of ligands with receptors, without the use of indirect labels, such as fluorescent markers and radioactive molecular tags. This label free direct sensing technique reduces the time and workload required to perform assays, and minimizes the risk of producing misleading results caused by molecular changes induced by the use of indirect labels. Another important aspect of the bio-sensing technique is that SPR based bio-sensing enables bio-molecular interactions to be measured continuously and in real-time, thereby enabling the determination of association and dissociation kinetic data in contrast to traditional “end point” analytical methods.
Over the past decade, interest in the unique optical properties of metallic nanoparticles has increased considerably with respect to the use of suspensions and films incorporating these nanoparticles for the purposes of exciting surface plasmons to enable the detection of SPR spectra. In addition, Surface Enhanced Raman Spectroscopy (SERS) for infrared absorbance spectral information and surface enhanced fluorescence for enhanced fluorescence stimulation can also be detected. Nanoparticles are particles that are less than 100 nanometers in diameter. Metallic nanoparticles display large absorbance bands in the visible wavelength spectrum yielding colorful colloidal suspensions. The physical origin of the light absorbance is due to incident light energy coupling to a coherent oscillation of the conduction band electrons on the metallic nanoparticle. This coupling of incident light is unique to discrete nanoparticles and films formed of nanoparticles (referred to as metallic island films). Achieving SPR with ordinary bulk materials requires the use of a prism, grating, or optical fiber to increase the horizontal component of the incident light wave vector (i.e., to achieve the required coupling).